In order to give a precise cylindrical finish to a shaft such as a driveshaft it is necessary to hold the semifinished shaft at two locations for rotation about its axis, apply torque to the shaft to rotate it, and press a finishing tool or stone radially inward against it. The workpiece must be supported quite solidly, so that its axis does not move as it is rotated, since the tool must press against it with enough force to grind or mill off high spots.
The standard procedure either uses lathe-type centers between which the workpiece is engaged axially, or chucks with three or more jaws that radially engage the workpiece surface. Both systems require extremely careful setup, with the holder axes aligned as perfectly as possible with that of the workpiece. A deformation in the workpiece at the held location can put the entire grinding operation offcenter. Similarly such mechanical gripping of the workpiece inherently adds any misalignment or irregularities in the holders to those of the holder, often compounding the problem of producing a cylindrically true finished workpiece. The holding force, whether axial or radial, is normally substantial, so that this force can frequently deform a workpiece which takes on a nonround shape when it relaxes when dechucked or otherwise removed from its holder. If a shaft is to be used at high rotary speeds, it is essential that it be very accurately machined on center, so that the time spent properly setting up and finishing the workpiece can be very large.
It has been suggested in German Pat. No. 2,407,261 to machine the end of a valve plunger or roller-bearing race by fitting the workpiece via an elastic ring or layer over a drive member and centering it loosely within a cylindrical guide. A liquid is fed under pressure to the space between the inner surface of the guide and the outer surface of the workpiece to center same within each other. This type of arrangement is only considered useful in superfinishing small annular and/or short workpieces.